Position detecting transducer

ABSTRACT

POSITION DETECTING TRANSDUCER FOR MEASURING ACCELERATION HAVING A CONDUCING PLANAR ELEMENT AND MEANS FOR MOUNTING SAID PLANAR ELEMENT FOR ANGULAR MOVEMENT. A MAGNET IS PROVIDED AND A COIL IS DISPOSE IN THE MAGNETIC FIELD OF SAID MAGNET AND IS SECURED TO SAID PLANAR ELEMENT FOR ANGULAR MOVEMENT WITH SAID CONDUCTING PLANAR ELEMENT. AN INDUCTIBE PICKOFF ELEMENT CONSISTING OF A PAIR OF COAXIAL WINDINGS IS PROVIDED WITH THE WIND INGS LYING IN A PLANE SUBSTANTIALLY PARALLEL TO THE PLANE OF THE CONDUCTING ELEMENT AND ON ONE SIDE OF THE CONDUCTING ELEMENT. MEANS IS PROVIDED FOR APPLYING EXCITATION TO THE PICKOFF TO ESTABLISH A MAGNETIC FIELD WITH THE CONDUCTING ELEMENT BEING POSITIONED RELATIVELY CLOSE TO THE PICKOFF ELEMENT SO THAT THE COUPLING BETWEEN THE WINDINGS IS DISTURBED AND LOSSES ARE REFLECTED INTO THE WINDINGS BY THE CONDUCTING ELEMENT. DETECTOR MEANS IS CONNECTED TO SAID MEANS FOR SUPPLYING EXCITATION TO THE PICKOFF ELEMENT TO GIVE AN INDICATION OF THE POSITION OF THE CONDUCTING ELEMENT RELATIVE TO THE PICKOFF. FEEDBACK MEANS IS COUPLED TO SAID DETECTOR MEANS FOR APPLYING A CURRENT TO SAID COIL PROPORTIONAL TO THE DISPLACEMENT OF THE CONDUCTING PLANA ELEMENT. THE MEANS FOR APPLYING EXCITATION TO THE INDUCTIVE PICKOFF ELEMENT CONSISTS OF AN OSCILLATOR THE PICKOFF ELEMENT TO ESTABLISH A MAGNETIC FIELD WITH THE CLUDES A TANK CIRCUIT AND FEEDBACK CIRCUIT. ONE OF THE WINDINGS OF THE INDUCTIVE PICKOFF ELEMENTS IS CONNECTED INTO THE TANK CIRCUIT OF THE OSCILLATOR TO FORM A PART OF THE TANK CIRCUIT AND THE OTHER OF THE WINDINGS IS CONNECTED INTO THE FEEDBACK CIRCUIT OF THE OSCILLATOR. THE OUTPUT OF THE OSCILLATOR IS MODULED BY VARIATIONS IN SPACING BETWEEN THE CONDUCTING ELEMENT AND THE INDUCTIVE PICKOFF ELEMENT. ADDITIONAL FEDBACK MEANS COUPLES THE DETECTOR MEANS TO THE OSCILLATOR TO CHANGE THE GAIN OF THE OSCILLATOR TO THEREBY CONTROL THE LOOP GAIN AND THE FREQUENCY RESPONSE OF THE TRANSDUCER.

4 Sheets-Sheet 1 Original Filed Feb. 4 1959 OUTPUT AMP DETECTOROSCILLATOR PICKOFF DEVICE MOVING a RESTORING SYSTEM 9% O O O O m w 5 wIll-ll ullnlll. 'l-lll .llll-J n DIDIII u M FL M n Flllllllllllllllllllllllllllllll I| Illi "M u 7m. II\J 6 I 2 5 8 a :A /9I. w v I. 9 4 5 X 5 5 9 5 n 4 ,J \IF I R or 0 W m m w M m Dec. 12 1972 4Sheets-Sheet 2 Original Filed Feb. 4

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1972 H. D. MORRIS POSITION DETECTING TRANSDUCER Original Filed Feb. 4,1959 4 Sheets-Sheet 3 7% m 4 T B m i T m 7 6 m m f R am N 6 F 8 R m A lm. C S O 0 B I w r\. WE M Om GR K .1 O u w P 5 MM INVENTOR.

Harold D. Morris Attorneys ig. I2

Dec. 12, 1912 H M Re. 27,532

POSITION DETECTING TRANSDUCER Original Filed Feb. 4. 1959 4 Sheets-Sheet4 INVENTOR.

Harold D. Morris Attorneys United States Patent Ofilice Re. 27,532Reissued Dec. 12, 1972 27,532 POSITION DETECTING TRANSDUCER Harold D.Morris, Pleasant Hills, Califi, assignor to Systron-Donner Corporation,Concord, Calif. Original No. 3,074,279, dated Jan. 22, 1963, Ser. No.794,487, Feb. 4, 1959. Application for reissue Apr. 24, 1970, Ser. No.31,578

Int. Cl. G01p 15/08 U.S. Cl. 73-517 R 20 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE Position detecting transducer for measuringacceleration having a conducting planar element and means for mounntingsaid planar element for angular movement. A magnet is provided and acoil is disposed in the mag netic field of said magnet and is secured tosaid planar element for angular movement with said conducting planarelement. An inductive pickofi element consisting of a pair of coaxialwindings is provided with the windings lying in a plane substantiallyparallel to the plane of the conducting element and on one side of theconducting element. Means is provided for applying excitation to thepickofi element to establish a magnetic field with the conductingelement being positioned relatively close to the pickofi element so thatthe coupling between the windings is disturbed and losses are reflectedinto the windings by the conducting element. Detector means is connectedto said means for supplying excitation to the pickofi element to give anindication of the position of the conducting element relative to thepickofi. Feedback means is coupled to said detector means for applying acurrent to said coil proportional to the displacement of the conductingplanar element. The means for applying excitation to the inductivepickofi element consists of an oscillator which normally oscillatescontinuously. The oscillator includes a tank circuit and a feedbackcircuit. One of the windings of the inductive pickofi element isconnected into the tank circuit of the oscillator to form a part of thetank circuit and the other of the windings is connected into thefeedback circuit of the oscillator. The output of the oscillator ismodulated by variations in spacing between the conducting element andthe inductive pickofi element.

Additional feedback means couples the detector means to the oscillatorto change the gain of the oscillator to there- This invention relates tostatic and dynamic position detecting transducers and more particularlyto such transducers commonly referred to as accelerometers of thelinear, angular and resonant types.

Heretofore, strain gauge accelerometers have been utilized. However, theuse of such accelerometers is often objectionable because they have alow impedance and a low output. Variable reluctance accelerometers havealso been used which give a modulated A.-C. output which may be of highlevel. However, the use of this type is also often objectionable becauseit suffers from reaction forces on the moving system which affect theoutput. Both of these types, the strain gauge and variable reluctance,when used in open loops have an output which is directly proportional tothe excitation so that the sensitivity of the accelerometer is directlyrelated to the power supply voltage level. This is a very undesirablefeature which can only be avoided by utilizing a closed loop. Closingthe loop on such accelerometers to provide a satisfactory device hasbeen found to be difiicult and expensive. Such accelerometers are undulycomplicated and still do not have the desired accuracy. It is apparent,therefore, that there is a need for position detecting transducers whichare compact, and simple, and which at the same time have high accuracywith a high level output.

In general, it is an object of the present invention to provide aposition detecting transducer which has high accuracy with a high leveloutput and which at the same time is relatively compact and simple.

Another object of the invention is to provide a transducer of the abovecharacter which is not alfected by the power supply voltage level.

Another object of the invention is to provide a transducer of the abovecharacter in which the output voltage is representative of the totalforces acting on the system.

Another object of the invention is to provide a transducer of the abovecharacter which can be utilized as angular, linear and resonantaccelerometers.

Another object of the invention is to provide a transducer of the abovecharacter which has a constant axis of sensitivity.

Another object of the invention is to provide a transducer of the abovecharacter in which the position of a dynamic or static element isdetected.

Another object of the invention is to provide a transducer of the abovecharacter in which the element being sensed is in the form of a flatplanar conducting member whose position is detected by a pancake typetransformer.

Another object of the invention is to provide a transducer of the abovecharacter in which the coil is excited by an oscillator and the outputof the oscillator is modulated by the position of the element.

Another object of the invention is to provide a transducer of the abovecharacter in which the use of a high carrier frequency in the oscillatoris made possible by the use of the alpha gain of a transistor.

Another object of the invention is to provide a transducer of the abovecharacter in which the tank circuit is in the collector circuit of thetransistor.

Another object of the invention is to provide a transducer of the abovecharacter in which the output of the amplifier is sufiicient to directlydrive the restoring mechanism.

Another object of the invention is to provide a transducer of the abovecharacter in which the current which flows in the amplifier preciselyrepresents the acceleration acting on the moving system.

Another object of the invention is to provide a transducer of the abovecharacter in which the output may be used directly to operate a recorderor to provide a high voltage output level.

Another object of the invention is to provide a transducer of the abovecharacter which can be automatically zeroed.

Another object of the invention is to provide a transducer of the abovecharacter which can be utilized in both closed loop and open loopaccelerometers.

Another object of the invention is to provide a transducer of the abovecharacter which can be utilized in accelerometers having either linearor angular degrees of freedom for the purpose of measuring linear and/or an gular accelerations.

Another object of the invention is to provide a transducer of the abovecharacter which has a relatively low impedance level and which will notbe affected by the presence of media in the surrounding area having ahigh dielectric constant.

Another object of the invention is to provide a transducer of the abovecharacter in which the pickofi can be isolated or physically separatedfrom the electronics.

Another object of the invention is to provide a transducer of the abovecharacter in which the circulating currents in the tuned circuit for theoscillator are effectively isolated.

Additional objects and features of the invention will appear from thepreferred embodiments which are set forth in detail in the accompanyingdrawings.

Referring to the drawings:

FIGURE 1 is a block diagram of a position detecting ;ransducerincorporating the present invention.

FIGURE 2 is schematic diagram of the position de tecting transducershown in FIGURE 1 being used as m accelerometer, to sense either linearand/or angular accelerations.

FIGURE 3 is a greatly enlarged plan view of the moving and restoringsystem and the pickoff device suitable for such an accelerometer.

FIGURE 4 is a side elevational view looking along the line 4-4 of FIGURE3.

FIGURE 5 is an enlarged top plan view of the pan- :ake type pickolftransformer and its support.

FIGURE 6 is a cross sectional view taken along the line 55 of FIGURE 5.

FIGURE 7 is a graph showing curves which represent be output of theoscillator at two different levels.

FIGURE 8 is a graph showing curves of the output of the detector at twodifferent levels of output of the oscillator.

FIGURE 9 is an enlarged cross sectional view of the moving and restoringsystem and a pickofif device suitable for an accelerometer with a singlelinear degree of freedom, for sensing linear accelerations.

FIGURE 10 is a cross sectional view taken along the line 1010 of FIGURE9.

FIGURE 11 is an enlarged view, partly in cross section, of the movingand restoring system and pickolf transformer.

FIGURE 12 is a block diagram of another embodiment of the presentinvention showing a transducer of the resonant type.

FIGURE 13 is an enlarged side elevational view of the moving andrestoring system and pickoff device suitable for use with a resonanttransducer.

FIGURE 14 is a cross-sectional view taken along the line 14-14 of FIGURE13.

In general, the present invention consists of a transformer with primaryand secondary windings in the form of a pancake and in which oscillatormeans is utilized for supplying energy to the primary winding toestablish a magnetic field. The secondary winding is conuected to theoscillator and supplies energy for oscillation. A planar conductingelement is positioned in the magnetic field in such a manner that itwill interrupt the maximum number of the lines of force. It serves tovary the coupling between the secondary and primary windings and thelosses which are reflected into the oscillator. Variations in thespacing between the element and the transformer serve to modulate theoutput of the oscillator. This modulation is detected by a detector andchanged into a high level current signal which is utilized for operatingan output amplifier. The output of the amplifier can be used for variouspurposes. [t can be used to operate a restoring mechanism so that thetransducer can be used as a closed loop accelerometer. Thus, upondeflection of the conducting element by acceleration, an automaticbalance between the input force provided by the restoring mechanism andthe force of the acceleration is established. The restoring current inthe restoring coil of the restoring mechanism is utilized for developinga voltage-across a fixedload resistor which is a precise measure of theinput acceleration. The output of the amplifier in another embodiment isused for feeding energy back into the oscillator to make possibleautomatic zeroing with reduction of the output of the position detectorresulting from rapid changes of position. A restoring force is appliedby other means than through servo feedback, so that displacements of themechanically resonant system from the average position, are detected.

In FIGURE 1, I have shown in block diagram a position detectingtransducer which is normally called an accelerometer. As shown in theblock diagram, such a transducer consists of a moving and restoringsystem 10 and a pickoff device 11. The pickoff device is connected to anoscillator 12. The output of the oscillator is fed into a detector 13which has its output amplified by an output amplifier 14. The outputcurrent from the amplifier 14 is fed through a load resistance R andthrough a feedback path 16 to the moving and restoring system 10. Anoutput voltage is generated across the output terminals 21 and 22 and islabelled as E Under action of acceleration, a force is generated on themoving system which tends to develop a displacement. As movement takesplace, the oscillator, detector and output amplifier 14 serve as aposition error detector and servo amplifier and generate a rapidlyincreasing feedback signal which is returned as current through the path16 to the restoring system. With a minute deflection of the movingsystem, the electro-mechanical servo action of the accelerometer resultsin automatic balance between the input force proportional toacceleration and the feedback force proportional to current in therestoring coil of the accelerometer. The voltage E developed across thefixed load resistance R is the electrical output of the instrument andis a precise measure of the input acceleration.

In FIGURE 2 is shown a schematic diagram of the accelerometer shown inFIGURE 1. The moving and restoring system 10 and the pickoif device 11are shown schematically in FIGURE 2 and will be described in detailhereinafter. Briefly, however, the moving and restoring system consistsof a moving coil 26 positioned between north and south pole pieces 27and 28 of a magnet 29. A hairspring 30 is utilized for supplying thecurrent to the moving coil 26 from the feedback path 16. An arm 31 iscarried by the moving coil 26 and moves with the coil. The free end ofthe arm is provided with a conducting element 32 in the form of a flatvane or paddle. The conducting element 32 is placed in such a positionthat it will interrupt the maximum number of lines of force from thetransformer 33. The transformer is constructed in a particular manner ashereinafter described and, in general, consists of a primary winding 34and a secondary winding 35.

The oscillator 12, the detector 13 and the output amplifier 14, ingeneral, include transistors X X X and X each of which is provided withthe conventional base, collector and emitter elements numbered 1, 2 and3. In particular, the oscillator 12 consists of the transistor X whichhas its base 1 connected to one side of a capacitor 36 by a conductor37. The other side of capacitor 36 is connected to one side of thewinding 34 by a conductor 38. The other side of the winding 34 isconnected to one side of a capacitor 39 by a conductor 40 and the otherside of the capacitor is connected to the collector 2 of the transistorX by a conductor 41. The collector 2 of the transistor X and theconductor 41 are connected through RF choke 43 and to a suitable sourceof positive D.-C. such as 15 volts D.-C. by a conductor 44. Conductor 41is also connected to a pair of diodes 46 and 47 by a conductor 48through a coupling capacitor 49.

The winding 35 has one side connected to a capacitor 50 by a conductor51. The other side of the capacitor 50 is connected to the emitter 3 ofthe transistor X by a conductor 52. The other side of the Winding 35 isconnected to the conductor 38 by conductor 53. A capacitor 54 isconnected between conductors 40 and 51;

Conductor 37 is connected by a conductor 55 to one side of a resistor56, and the other side of the resistor 56 is connected to the conductor44 by a conductor 57. Conductor 52 is connected through a resistance 58to a suitable source of negative voltage such as volts D.-C. by aconductor 59. A diode 61 is connected between conductor 59 and conductor55. An RF bypass capacitor 62 is connected around the diode 61.

An RF bypass capacitor 63 is connected between conductors 44 and 59. Aresistant 64 has one end connected to the conductor 38 by a conductor 66and the other end is connected to one side of the resistance R by aconductor 67. The other side of the resistance R is connected to groundas shown. Conductor 67 is connected by a conductor 68 to one side of thewinding 26. The other side of the winding 26 is connected by a conductor69 to the emitter 3 of a transistor X A capacitor 70 is connectedbetween the conductor 67 and ground. Conductor 69 is also connected toone side of a resistance 72 and the other side of the resistance isconnected to the collector 2 of a transistor X by a conductor 74. Thecollector 2 of the transistor X is connected to the conductor 44, andthe base 1 of the transistor X is connected to one side of a resistance76 by conductor 77. The other side of the resistance 76 is connected tothe conductor 44. Conductor 77 is also connected to one side of a diode78 and the other side of the diode is serially connected to anotherdiode 79. The other end of the diode 79 is connected to the conductor 74by a conductor 81. The emitter 3 of the transistor X is connected to theconductor 59 and the base 1 of the transistor X is connected to theemitter 3 of a transistor X The collector 2 of the transistor X isconnected to conductor 77 by conductor 82. The base 1 of the transistorX is connected to the other side of the diode 46 by a conductor 84. AnRF bypass capacitor 86 is connected between the conductor 84 and theconductor 59. The other side of the diode 47 is connected to theconductor 59.

Operation of the accelerator shown in FIGURE 2 may now be brieflydescribed as follows. In general, the pickup transformer 33 detects theposition of the conducting element 32. The transformer 33 is excited bythe oscillator. The oscillator output is modulated by the yariation inspacing between the conducting plane 32 and the transformer 33. This isdetected and changed into a high level current signal which operates thetransistors in the output amplifier. The output amplifier brings thepower level up to where the restoring mechanism will automatically beregulated by the closed servoloop so that the current which flowstherein actually represents the acceleration acting on the conductingelement 32. Therefore, the output current will be a function not of anyresistance in the output circuit, but only of the acceleration which thesystem is undergoing. This output current may be used directly tooperate a recorder or the voltage across the resistance R may beutilized. The presence of the resistance R does not affect the actualoutput current of the servo but it will affect the total loop gain. Thisis true because one of the transfer constants in the loop of theaccelerometer is the output circuit where the output voltage from theamplifier is changed into an output current by the total resistance inthe output circuit.

The oscillator 12 utilizes the alpha gain of the transistor X ratherthan the beta gain because it employs a grounded base circuit. Thismakes possible the use of a higher carrier frequency. The Winding 34,winding 35, and the capacitor 54 form the tank circuit for theoscillator and the capacitor 54 is tuned for the desired frequency.However, since the exact frequency of the oscillator is not important, afixed capacitor is utilized rather than a variable capacitor. The fixedcapacitor is chosen so that the oscillator will operate at a suitablefrequency such as, for example, 1 /2 megacycles The choice of thefrequency is made so that the components are of optimum size and withinthe bandwidth of the transistor. As is well known to those skilled inthe art, the higher the frequency, the smaller the components. Sincesmall components are generally desirable in devices of this type, higherfrequencies are normally utilized. However, the higher the frequency,the higher the bandwidth required of the transistor. The bandwidthdesired should be below the alpha cutoff to prevent a serious drop incurrent efficiency.

Any suitable transistor may be utilized in the oscillator. A transistoridentified by type No. 2N332/2Nl17 operates very satisfactory because ithas been found that transistors having low beta, that is, low gain,operate more satisfactorily than those with high beta because the alphacutoff or frequency response has been found to vary inversely with thebeta of the transistor, that is, the higher the gain of the transistor,the lower the frequency response.

The tank circuit for the oscillator is shunt fed and for that reasonthere is no D.-C. potential between the pickoff transformer 33 andground due to the oscillator positive supply voltage.

The RF choke 43 serves to isolate the plate supply or the positive D.-C.voltage appearing on line 44 from the collector circuit of theoscillator. The capacitor 39 couples the collector circuit to the coil34 without allowing any appreciable D-.-C. voltage to appear at thepickoff transformer.

The voltage applied to the emitter-base circuit of the transistor X isstabilized by the diode 61. The diode 61 is of a particular type knownas a Zener diode and identified by type No. ZA-l5. As is well known tothose skilled in the art, the Zener diode has a specific breakdownvoltage in the reverse direction. Thus, in the present circuit, itbreaks down from the voltage applied from the conductor 44 and maintainsa constant voltage thereafter independent of the supply voltagesupplying the base-emitter circuit of the transistor X Since the actualvoltage appearing between the base and the emitter is relatively small(e.g., approximately /2 volt), and because very little current isrequired by the base when the emitter current is flowing, most of thevoltage from the conductor '44 appears across the isolating resistance 5which thereby establishes a constant biasing current flowing into theemitter of the transistor X Substantially this same current flows in thecollector circuit of the transistor because very little is contributedby the base of the transistor. Stabilization of the collector current ofX as described makes the output of the oscillator substantiallyindependent of supply voltages.

The pickolf transformer 33 is capacitively coupled to the oscillatorcircuit by the capacitors 36, 39 and 50. Capacitor 54 is mounted closeto the pickoif transformer when separation of the pickoff andelectronics is necessary, so that RF circulating currents do not flowthrough the capacitors, 36, 39, and 50, thereby minimizing the voltagedrop in the capacitors as well as the magnetic fields surrounding theconductors. The position of capacitor 54 effectively isolates thecirculating current of the tuned circuit so that voltage drops andcouplings will not interact to provide spurious modes of oscillation.The resistor 64 connecting one output terminal to the pickofftransformer allows the pickoff transformer to carry the averagepotential of the output. This eliminates the major source ofnonlinearity, that due to coulombic forces resulting from potentialdifference between the pickoff and the moving system.

A pickoff of this type has a relatively low impedance so that it is notmarkedly affected by the presence of a medium having a high dielectricconstant. This is important when it is desirable to separate the pickofffrom the electronics or when the pickoff is used in oil filledinstruments.

The voltage appearing across the winding 35 is essenially applied fromthe base to emitter on the transistor so that the RF voltage appearingon winding 35 modlates the DC. emitter current. Therefore, RF currents 1the winding 35 will flow directly in the emitter ciruit and will becarried straight through to the collector 1 the same magnitude with onlya small loss due to the ase being intermediate. Therefore, modulation ofthe mitter current by the winding 35 also causes modulalon of thecollector current. This collector current is oupled through thecapacitor 39 into the transformer 3.

The oscillator itself works in a manner which is similar I) theArmstrong tickler oscillator. Instead of a part f the output voltagebeing fed back, a part of the outut current is fed back to the input.The winding 35 wound so that it changes the amplitude and phase f thecurrent to feed it back into the input in the proper -hase.

The transformer 33 provides a majority of the current aim in the totalcircuit. Assuming that there is no :akage in the transformer, thetransformer 33 should ,ive a gain which is directly proportional to theratio f turns of the winding 34 to the turns of winding 35. Thus, theratio of the current in the collector circuit with espect to the currentflowing in the emitter circuit will re determined by this ratio. Thetransistor X is, thereore, not being used as a current gain device butactually .s a voltage gain device to drive the high impedance ankcircuit connected to the collector.

This oscillator circuit has a distinct advantage in that t has asubstantially constant gain regardless of temaerature changes. As iswell known to those skilled in he art, in the normal transistor, if onlythe current gain f the transistor were used, the gain would drop offsharpy with a drop in temperature. On the other hand, the 'oltage gainof the transistor increases with a drop in emperature. In thisoscillator circuit, these two char- .cteristics are combined because theimpedance of the vinding 35 operates into the very low impedance of the=mitter circuit.

As is well known to those skilled in the art, when an =xtremely lowimpedance winding is used for the emitter lrive, the voltage gain of thetransistor is being used achieve oscillation whereas when a very highimpedance vinding is used for the emitter drive, the current gain of hetransistor is being used to achieve oscillation. I have letermined thatwith an intermediate impedance, the chartcteristic of the pickup remainsunchanged through wide emperature ranges (-S C. to +125 C.) without'equiring compensation. This is due to the fact that vhen thetemperature rises, the current gain increases llld the voltage gaindecreases, and when the temperature lrops, the current gain decreasesand the voltage gain ncreases.

As explained previously, the diode 61 serves to mainain the operatingpoint of the transistor X as the tem- Jerature changes because there islittle change in the inernal voltage of the transistor (approximately /2volt) :0 that the total operating current of the transistor renainsessentially constant with temperature.

Thus, to obtain a device which will operate satisfacorily in wide rangesof temperaure, it is only necessary 0 choose a proper impedance for thepickup winding 35. For example, with the transistor type No. 2N332/ZN117, it was found that an impedance of 20 ohms gave Iery satisfactoryresults.

The capacitor 49 couples the collector circuit of the )scillator over tothe full wave detector 13. The full wave detector consists of the diodes46 and 47 which ire connected in opposite directions. The full wave de-1ector acts in a manner similar to a conventional voltage loublercircuit in that the capacitor 49 charges through the diode 47 fornegative going parts of the sine wave and discharges through diode 46for the positive going parts so that a current of essentially doubleamplitude is being sent through the diode 46 and into the preamplifiertransistor X which is part of the output amplifier 14. The capacitor 86acts as an RF. bypass to minimize the pulsing of the current flowinginto the preamplifier transistor X Capacitor 63 is an R.F. bypasscapacitor and serves to prevent external radio frequencies fromalfecting the internal circuitry and prevents leakage from the internaloscillator out through the power supply.

The detector current which is supplied by the detector 13 is multipliedby the beta of the transistor X and is fed into the base of thetransistor X which through the diode-resistor network 72, 78 and 79controls the transistor X The voltage appearing across the resistor 72provides a voltage for operating the diodes 78 and 79.

Transistor X, has a bias current flowing from the B+ line 44 through theresistance 76 to its base. When this bias current is allowed to flow,the transistor X is pulled on vigorously. An alternate path is providedfor the bias current through the diodes 78 and 79 so that the transistorX can bypass the control current for the transistor X and turn it off.

For negative going signals, transistors X and X, are carrying currentand supplying the signal current. Transistor X is almost entirely turnedoff. The current flows from ground through the load resistance R theconductor 68, the torque coil 26, the conductor 69, resistance 72, downinto the transistor X For large positive going signals, transistors Xand X run at a low current level and transistor X4; is turned on by theunshunted bias current flowing through resistance 76. This pulls theoutput up to a maximum positive level. The current fiows throughtransistor X through conductor 69, the torque coil 26, conductor 68, aresistance R to ground. This arrangement makes it possible to havemaximum effort when the signal goes positive and negative for moving thetorque coil 26.

The capacitor acts as a phase compensation capacitor and is utilized forstabilizing a gas-filled or evacuated accelerometer. In the aboveaccelerometer, the loop is essentially very unstable because it is asecond order system with very low inherent damping, that is, there areno losses in the circuit. In order to stabilize the loop, aleading-phase current is provided by the shunt path across the loadresistor R This leading current acts as velocity damping and is selectedto provide optimum damping to the servo.

As an alternate, a fixed resistor can be used to replace the transistorX This, however, would not be nearly as efficient as the transistor, andwould result in a much lower effective loop gain.

The output amplifier 14 which consists of the combination of thetransistors X and X is basically an emitter follower operating into agrounded emitter stage. The transistor X makes possible a very goodutilization of the available power supply voltage in that it permits thesignal from the double transistor amplifiers X and X to go almost as farnegative as the available negative supply voltage. Because thetransistor X derives its collector current from the base 1 of transistorX.,,, the transistor X can very effectively assist in turning ofitransistor X and make it possible to very closely approach the negativevoltage on line 59.

The network controlling transistor X; has been arranged so that thevoltage across resistor 72 is maintained substantially constant. Thus,except for a variable current flowing through the resistance 76, thetransistor X operates at a constant current level irrespective of thelevel of the output voltage. Therefore, to the first approximation,there is very little change in the current required on transistor X inorder to attain any desired level of output between a certainpredetermined range, as for example, :10 volts.

By way of example, one embodiment of the present invention had thefollowing values for the various components.

Turns Winding 34 50 Winding 35 Capacitors:

36 mf .02 39 mf .01 49 mmf 220 50 mf .005 54 mmf 300 62 mf .01 63 mf .0170 mf .15 86 mf .001

Value chosen for optimum response.

RF Choke 43 microhenries 500 Resistors:

56 ohms 15K 58 ohms K 64 megohms 10 72 ohms 270 76 ohms K R ohms 5000Transistors:

X X X and X Type No. 2Nl17 46, 47, 78 and 79 Type No. 1N625 61 Type No.ZA-15-2 One moving and restoring system 10 and pickolf device 11 foundto operate very satisfactorily in the accelerometer shown in theschematic diagram in FIGURE 2 is shown in detail in FIGURES 3, 4, 5 and6. As shown therein, the transformer 33 is in the form of a flat pancake so that the greatest number of lines are interrupted by the paddleor vane-like conducting member of element 32. The transformer is mountedon a block 91 of suitable insulating material such as Micarta.

In fabricating the small transformer 33, it has been found desirable toprovide the block 91 with a cylindrical extension 92 centrally locatedon the block and extending from the top surface of the block a suitabledistance such as .007 of an inch. The transformer 33 consisting of theprimary and secondary windings 34 and 35 can then be wound on theextension 92 in any suitable manner. For example, a washer (not shown)can be mounted on the extension 92 by a screw (not shown) and then thewire forming the transformer can be wound into the small groove betweenthe washer and the block 91. The secondary winding 35 can be wound firston the extension 92 and then the primary winding 34 can be wound on theextension. By winding the wires forming the transformer in this manner,it is possible to fabricate the transformer so that it takes the desiredpancake form on top of the block 91. During the winding of the wiresonto the extension 92, they can be coated with an epoxy material whichis allowed to harden before the washer is removed. Thus, when the washerwas removed, a perfectly flat face is presented by the top of thepancake type transformer 33.

The transformer 33 can be formed from any suitable material. Forexample, it was found desirable to fabricate the secondary winding 35with 5 turns of No. 45 Formvar copper wire. The primary Winding 35 wasformed with 50 turns of similar wire. The ends for the respectivewindings were brought down into the block 91 and then out of the blockfor connection to the other circuitry.

The pancake type construction is desirable because a doughnut-shapedfield is obtained with a maximum field strength in the internal openingof the pancake. The small secondary winding 35 is transformer coupled bythe lines of force to the primary winding 34. The current flowing in thesmall Winding 35 will generate a field which opposes the field of thelarge primary winding 34. When the conducting plane 32 is brought closeto the pancake transformer 33, the conducting plane 32 tends to appearlike a lossy shorted turn on the major or primary winding 34.

It is desirable that the vane-like or paddle-like member 32 be asubstantially planar member formed of a conducting material such asaluminum. Other materials, including copper, brass and steel, may beused. The conducting plane can be of any suitable thickness such as .008of an inch of any suitable size. However, in order to achieve maximumelficiency, the conducting plane should have a larger size than the sizeof the pancake transformer 33 so that edge effects are minimized. Forexample, in one embodiment of the invention, the transformer 33 had anOD. of .250 and an ID. of .187 of an inch, and the conducting plane 32had a diameter of .350 of an inch.

With such parameters, it was found that the normal operating positionfor the conducting plane 32 was between approximately .007 to .014 of aninch. This means that the conducting plane is generally between 1 and 2thicknesses of the transformer away from the transformer 33 and gives anindication of the large effect movement of the conducting plane 32 hason the field of the transformer 33. A substantially linear voltageoutput of 10 to 20 volts can be obtained for .001 of an inchdisplacement of the conducting plane. Up to 10 volts has been realizedfor a movement of .00001 of an inch.

The effects of the conducting plane 32 nearing the transformer 33energized by the oscillator 12 are twofold. One effect is the reflectionof losses into the ocillator, and the other is the shielding effect. Theconducing plane reflects losses because it is not a perfect conductor.It acts as a shield because it distorts the shape of the field createdby the primary winding and thereby changes the effective couplingbetween the primary and secondary windings of the transformer. For thatreason, the closer the conducting plane 32 comes to the trans former 33,the less the coupling between the primary and secondary windings. Theamount of energy that the collector circuit of the oscillator feeds intothe emitter circuit is varied by the coupling. The loading in thecollector circuit is varied in the same direction by the variations inthe reflected losses caused by movement of the conducting plane.

Movement of the conducting plane -'32 relative to the transformer 33,therefore, effects a variation in the loss reflected and a variation inthe coupling. Since both of these effects are additive, they serve tocreate a very sensitive device. The construction hereinbefore describedpermits the interruption of the maximum number of lines of force andthat maximum variation in reflected losses and maximum variation incoupling between the windings of the transformer and, therefore, alsohelps to create a very sensitive device.

The conducting plane 32 is carried by the arm 31 which is mounted on apivot 97 carried by upper and lower support means 98 and 99. The winding26 is adapted to rotate between north and south poles 27 and 28 of thepermanent magnet 29 which is of conventional construction. The permanentmagnet 29 is enclosed within a soft iron shield 102 which is carried bya mounting 103 also of conventional construction.

In FIGURES 7 and 8 are shown typical performance curves for anaccelerometer of the type hereinbefore described. FIGURE 7 shows theoutput voltage at the collector of the oscillator 12 with the diodedetector 13 disconnected. This output voltage is plotted against thespacing of the conducting plane 32 from the pancake transformer 33. Twocurves 106 and 107 are shown. Curve 106 represents the output when thereis a high level of current flowing in the oscillator whereas curve 107represents the output when a low level of current is flowing in theoscillator. When there is a high level of current flowing in theoscillator, the conducting plane 32 must be moved in relatively close tothe pancake transformer 3-3 in order to kill the oscillations in theoscillator. Thus, it can be seen that the output voltage remains at zerountil there is a certain spacing between the conducting plane and thepancake transformer and only begins at a certain spacing designated as dand indicated as point 108 on the curve 106. At this spacing, theoscillator begins oscillating and the output voltage increases verysharply on a relatively steep curve as shown and continues on this steepcurve until a point 109 is reached, at which point secondary oscillationoccurs to cause a discontinuity in the curve. This discontinuity has noimportance in the range of operation of the present device.

The dotted line 111 represents the threshold voltage E which indicatesthe voltage required on the collector of the oscillator 12 before thedetector 13 begins to throw a current into the output amplifier 14 andinto the load resistance R;,. The spacing d represents the spacingrequired between the conducting plane 32 and the transformer 33 beforethe detector current as shown in FIGURE 8 will begin rising from zero.Point 112 indicates intercept on the curve 106 with the spacing d Thus,when the diode detector 13 is connected to the oscillator 12, the outputvoltage curve 106 rises from the point 108 to the point 112 asindicated. After point 112, the curve bends over until it is almost aflat line with no vertical rising character because the diodes of thedetector 13 serve to absorb all of the excess power that is being pulledinto the oscillator through the emitter circuit.

The oscillator 12 is not a variable level oscillator but rather avariable output oscillator because the change in RF voltage observed atthe collector of the oscillator is very small in comparison to thechange in the output of the detector as the conducting plane 32 is movedback and forth with respect to the transformer 33.

For the low level output of the oscillator 12 represented by the curve107, it can be seen that substantially the same results are obtained,with the exception that the gain of the oscillator is somewhat increasedunder a high current condition than on a low current condition. This isshown by the difference of the slopes of the two curves 113 and 114 inFIGURE 8. With respect to curve 107, oscillation begins at the spacing drepresented by the point 116 on curve 107 and also rises steeply asshown until a discontinuity at 117 occurs for the same reason that thediscontinuity 109 occurs in curve 106. The point 118 represents thepoint at which current begins to flow in the detector circuit and isrepresented by the spacing d The detector current is shown in curve 114.Since the curve 114 is less steep than curve 113, it can be seen thatthe gain of the oscillator is greater under the high current conditionthan under the low current condition. From the curves 113 and 114, itcan be seen that the detector current rises very sharply as soon as thethreshold voltage E is reached on the collector of the oscillator.

The difference in the slope of the two curves 113 and 114 is importantfor automatic zeroing of the accelerometer as hereinafter described.From these curves it can be seen that the spacing to obtain a certainvalue of current in the detector circuit can be changed merely bychanging the operating current of the oscillator.

The apparatus shown in FIGURES 4, and 6 is commonly associated with whatis called a pendulous or angular-motion accelerometer. In FIGURES 9, 10and 11, I have shown apparatus which is commonly associated with what iscommonly called a linear-motion accelerometer. As shown, this apparatusconsists of a cylindrical rod-like member 151 which forms a mass. It isdesirable to suspend this mass in such a manner that it has as muchfreedom as possible for movement along one axis, as for example, theaxis of the rod 151, and none along other axes. For that reason, it isdesirable to have a spring constant which is as close to zero aspossible. However, since the mass must be supported it is not possibleto obtain complete freedom of movement along the desired axis.

One method of mounting for the mass 151 consists of a pair of springmembers 152 and 153. Each of the spring members is provided with acentral opening 154 which accommodates the associated end of the mass151 and frictionally engages the mass. As shown, the spring member isprovided with a plurality of offset arcuate cut-out portions 155 isconcentric circles. Three ears 156 spaced apart are provided on each ofthe spring members and are mounted on screws 1'57 placed in the housing158. It has been found that this type of mounting for the mass providesmaximum constraint on the axes perpendicular to the desired line ofmotion for the mass and minimum constraint along the axes or desiredline of motion for the mass 151.

The housing 168 consists of a base 159, an intermediate cylindricalsection 161 and a cap 162. Certain of the screws 157 are utilized forsecuring the intermediate section 161 to the base 159 and certain of thescrews 157 are utilized for securing the cap 162 to the intermediatesection 161 as shown.

The pickoif device is mounted in the cap 162 and consists of acylindrical insulating member 163 which is centrally disposed in andheld by the cap 162. A pancake type transformer 164 similar to thetransformer 33 is mounted on the inner end of the insulating member 163.The windings of the transformer are connected to terminals 166 mountedin the cap.

A conducting plane 168 is mounted on the end of the rod 151 and issimilar to the conducting plane 32. It is mounted on the rod 151 atright angles to the rod and in a plane parallel to the plane of thetransformer 164 so that it also interrupts the maximum number of linesof force from the transformer 164.

Restoring means 171 is mounted in the cap 159 and consists of a magnet172. The magnet is formed with a base 173 upon which is mounted a barrel174. A cylindrical member 176 is mounted on the base 173 within thebarrel 174. The magnet is polarized in such a manner that thecylindrical member 176 serves as the north pole, and the barrel and baseserve as the south pole.

A cylindrical winding 178 is mounted on the end of the rod 151 which isadjacent the base 159 and encompasses the pole 176. The winding 178 isconnected to the terminals 179. The terminals 179 are normally connectedto the output from the output amplifier 14 in the same way that currentis fed to the moving coil or winding 26 in the angular-motionaccelerometer.

The principle of operation of apparatus of the type shown in FIGURES9-11 is similar to that of the apparatus shown in FIGURES 3 and 4,except that it operates with linear motion rather than angular motion.When connected to circuitry such as that shown in FIG- URE 2, a smalldisplacement of the plane 168 relative to the transformer 164 causes thegeneration of a feedback signal to the winding 178 of the restoringsystem 171. The transformer 164 determines whether the mass 151 istrying to move with respect to the case or housing 158. If the mass istrying to move, current is fed back into the coil 178 and tends tooperate on the mass 151- to force it back to its zero position.Therefore, another force balance system is provided which operates onprinciples similar to that hereinbefore described. As in theangular-motion accelerometer, the electro-mechanical servo actionresults in automatic balance between the input force proportional to theacceleration and feedback force proportional to the current in therestoring Winding 178.

The angular-motion accelerometer lends itself to certain applications,particularly because the meter-type mechanism utilized for the restoringmechanism is very small and has desirable characteristics. However, sucha system has an inherent disadvantage in that there is always somefriction between the pivots and the supports for the pivots which placesan indeterminate factor in the output fro-m the accelerometer due to thespurious torques acting directly on the moving system.

The linear-motion accelerometer avoids this difficulty by eliminatingfriction. However, such an accelerometer must work against a very heavyspring constant, and for that reason the gain of the system must beboosted substantially to overcome this heavy spring constant. Thisdisadvantage is far outweighed by the fact that a linear motion systemhas a constant axis of sensitivity, whereas the angular-motionaccelerometer relies upon electronics to maintain the axis ofsensitivity in some relation with the case of the instrument. Thelinear-motion accelerometer is not dependent in this way because theline of motion of the mass is determined only by the mechanical springsrather than by the electronics associated with the mechanism.

The accelerometers hereinbefore described are well adapted totelemetering, navigation, control gyro erection, and shorter rangeinertial guidance applications in aircraft and missiles. The outputcurrent from the accelerometer is proportional to acceleration and is ofsuflicient level to operate magnetic amplifiers, servo valves, and othercontrol components or instrumentation without further amplification.With single or double integration of accelerometer output, velocity anddisplacement can be measured on an inertial basis. Orthogonal componentsof acceleration, and magnitude and direction of total vectoracceleration may be measured by using three units.

The hereinbefore described embodiments have been found to have very lownoise level and to have the ability to sense very small changes inacceleration. Practical ernbodiments of my accelerometers have beenfound to have a resolution to 0.002% of full scale, linearity to 0.02%of full scale, and D.-C. reproducibility to 0.02% of full scale. Theyalso have zero output at zero input, a minimum full range of plus orminus 0.05 g. and a maximum full range of plus or minus 50 g. An outputup to :30 volts or :2 milliamperes can be obtained. This output has beenobtained with a displacement of less than .001 of an inch of theconducting element. In addition, the output is not dependent on inputvoltage regulation. The output voltage has been found to be proportionalto acceleration for frequencies from zero to 25 cycles per second andhigher. Useful outputs have been obtained up to several times thisfrequency with reduced amplitude accuracy and full scale range.

Another embodiment of the invention is shown in the schematic diagram inFIGURE 12, and consists of a moving and restoring system 180 and apickoif device 1-81 which are similar to those hereinbefore described.The pickolf device 181 is connected to an oscillator 1 86 and the outputof the oscillator is fed into a detector 187. The output of the detectorfeeds into the output amplifier 188. The oscillator, the detector andthe output amplifier are similar to that hereinbefore described and forthat reason will not be described in detail. The output is supplied tothe user at terminal 189.

A feedback path 181 is provided. The feedback includes an amplifier 192of conventional design which feeds into a filter 193. The filter 193consists of serially connected resistances 194 and 196 which have theircommon connection connected to ground through a capacitor 197. Theoutput of the filter is connected to the oscillator 186 and is connectedin the oscillator circuitry which is shown in FIGURE 2 at the feedbackfunction 198.

The filter 193 filters out the high frequencies and, therefore, preventsthe high frequencies from being fed back. In effect, it nullifies theeifect of the high frequencies on the oscillator 186. By highfrequencies, I mean those frequencies in excess of one cycle per second,for example. The filter 193, therefore, permits only the average offsetin the output 189 to be fed back to the oscillator. The current whichflows in the feedback path 191 actually adds to the current at thefeedback junction, that is, it is linearly summed with the constantcurrent from the bias source of the resistance 58 and, therefore, addsor subtracts from the average current operating level of the transistorand moves the characteristic of the detector back and forth, as forexample, between the curves 113 and 114 as shown in FIGURE 8. Thus, ineifect, the feedback path acts as an automatic centering means byproviding negative feedback, and serves to determine the operatingcurrent level of the oscillator and thereby the characteristic of thedetector as hereinbefore described in conjunction with FIGURES 7 and 8.

For example, let it be assumed that the pickoif device 1 81, which canbe called an error detector, is positioned near a surface or memberwhich it is desired to observe. This general positioning of the pickoifwith respect to the surface or member in effect is like a poorlypositioned pickoif with respect to its inherent electrical zero. Thefeedback path 191 will make a correction in the electrical zero byshifting the characteristic of the detector 187 so that the averageD.-C. output from the amplifier 188 returns to zero and thereby movesthe sensitive transducer position to the center of the operating range.Thus, in this manner, it is possible to close the loop on the detectorand at the same time maintain its average operating position in thecorrect position while observing the vibrations or other rapid changesin the position of the element being observed.

This can be done with an arbitrarily slow feedback path so that positionchanges of very slow character can be observed. If desired, anintegrator can be included in the feedback path to automatically nullthe output of the transducer until it is desired to observe a change inposition. The integrator input can then be disconnected and the feedbackpath can then maintain the electrical zero at the point determinedbefore the disconnection occurs. For example, in a practical embodiment,the pressure diaphragm in a rocket can be observed before the rocket isfired. Any output from the pickoff can be nulled out during the time therocket is fired and thereafter the automatic zeroing circuit disabled sothat any subsequent variations can be detected and changed intoelectrical signals regardless of their frequency components.

This combination of an automatically nulling pickoif with a mechanicallyresonant system permits the manufacture of resonant transducers whichcan detect extremely low amplitude vibrations at some specific frequencyto which the mechanical system is tuned. Thus, it is possible to detectvibrations expected in a gyro system due to drift or other sources.

By way of example, one practical resonant accelerometer embodying theabove invention had a clearly defined proportional response down toapplied displacements of 3 l0- radians double amplitude at its naturalfrequency of 850 cycles per second. The noise level of the transducerwas measured at 40 l0* radians double amplitude applied vibration at thenatural frequency. The mechanical Q was found to be approximately 800.

A moving and restoring system 180 and a pickolf device 181 suitable foruse in the resonant transducer shown in the schematic diagram in FIGURE12 is shown in detail in FIGURES l3 and 14. As shown therein, itconsists of a mass in the form of a cylindrical rotor 201 which issupported by a pair of torsion rods 202 and 203 in axial alignment withthe axis of the rotor 201. The ends of the torsion rods remote from therotor 201 are supported very firmly by suitable, relatively sturdysupport means such as the massive U shaped member 204 as shown in thedrawing. Such a massive support means is provided so that vibrations inthe mass 201 do not cause losses in he supporting means which wouldresult in a reduction )f the mechanical Q of the system.

The torsion rods 202 and 203 provide the restoring Force. The torsionrods have a constant stiffness which is letermined by theircharacteristics. Therefore, it is ap- Jarent that this type ofrestoration is quite dilferent from hat of the transducers hereinbeforedescribed in which he restoring torque is determined by the servo itself:hrough the position detector.

The moving system also includes a fiat conducting plane 206 which isfixed on the rotor 201 and extends radially :herefrom.

The pickofi device 181 is in the form hereinbefore de- ;cribed andconsists of a pancake type transformer 207 avhich is carried by amounting block 208 in such a nanner that the transformer 207 lies in aplane substan tially parallel to the conducting plane 206. When this:ransformer 207 is connected into the circuitry herein- Jefore describedin conjunction with FIGURE 12, the :ransformer 207 is provided withautomatic zeroing and )bserves the conducting plane 206. With suchautomatic zeroing, the average output from the circuit is zero. How-:ver, maximum sensitivity to vibrations or movement of the conductingplane 206 is provided.

It will be noted that a flat plate 211 has been mounted across the openpart of the U shaped member 204 and that affixed thereto there is amounting block 212 which :arries a pancake type transformer 213. A fiatconducting plane 214 is mounted on the rotor 201 and also extendsradially therefrom in a plane which is parallel the plane of theconducting plane 206. This additional pickofi device in the form oftransformer 213 has been provided to eliminate sensitivity to linearaccelera- ;ion. Without such additional pickotf device, the mass wouldbe deflected under linear acceleration and cause a response in thepickolf device. However, by utilizing pickoffs, as shown in thedrawings, on opposite sides of the inertial element, that is, the massor rotor 201, rotation of the moving system will bring the twoconducting planes closer to both the pickofi devices simultaneously,whereas a linear acceleration would deflect the mass without rotationand would bring one conducting plane closer to the pickoff and move theother conducting plane farther away from the pickolf. By properlyconnecting the pickofi devices, e.g., in serial aiding relationship, theeffect of the linear acceleration can be nulled out.

The moment of inertia of the rotor 201 connected together with thespring constant of the torsion rods determines the natural frequenciesso that the only real control necessary after choosing the size of therotor is to adjust the size of the torsion rods to tune the system intoresonance at the desired frequency. The restoring force on the rotor isalmost entirely due to the torsion rods and desirably so because themotions of all the other support members, since they are relativelymassive, are negligible, and therefore, their dissipation or effect onthe Q of the moving system is also negligible.

With such apparatus, it was found that mechanical Qs of 1000 can beeasily achieved and that Qs of over 10,000 are possible.

This system is almost a completely undamped system and is used so that avery high sensitivity can be obtained at the resonant frequency. In theangular accelerometer hereinbefore described, as much as possible of theresonant behavior is eliminated because a fiat response is desired fromD.-C. up to the maximum useable frequency, as for example, from D.-C. upto 100 cycles per second.

This particular arrangement which is shown in FIG- URES 13 and 14 isparticularly advantageous in that it makes possible a very compact andsub-miniature pickoif which is adaptable for remote attachment of theassociated electronics so that only the sensing element need be mountedWithin the device which is being observed, as for example, a gyro. Theoutput of the resonant unit, since the resonant frequency is known, canbe examined with a signal analyzer to eliminate noise frequencies whichmay interfere with the measurements desired. In a more elaborate system,it is possible to use the principles of cross-correlation to examine thenoise or output of the system and compare it with the referencefrequency so that small components of the resonant reference frequencycan be sensed and used as an indication of minute vibration.

It is apparent from the foregoing that I have provided a transducerwhich can 'be utilized for detecting static and dynamic positions ofelements and particularly a transducer of this type which is adaptableto be used in linear and angular-motion accelerometers of the resonantand non-resonant types. When utilized with transistorized electronics,the accelerometers are very small and compact, and they provide anoutput voltage which is sufficient for many purposes without furtheramplification. Accelerometers so constructed are particularly sensitiveso that very small changes in acceleration can be noticed.

It is also apparent from the foregoing that my transducer can beutilized in many different types of industrial regulating and controlapplications. For example, it can be used as a general purpose positiondetector in open or closed loops where generation of an error signal isrequired for follow-up or regulation.

I claim:

1. In a transducer for measuring acceleration, a conducting planarelement, means for mounting said planar element for angular movement, amagnet, a coil disposed in the magnetic field of said magnet and securedto said planar element for angular movement with said conducting planarelement, an inductive pickoff element consisting of a pair of coaxialwindings, the windings lying in a plane substantially parallel to theplane of the conducting element and on one side of the conductingelement, means for applying excitation to the pickoif element toestablish a magnetic field, the conducting element being positionedrelatively close to the pickofi element so that the coupling between thewindings is disturbed and losses are reflected into the windings by theconducting element, detector means connected to said means for supplyingexcitation to the pickolf element to give an indication of the positionof the conducting element relative to the pickofl, [and] feedback meanscoupled to said detector means for applying a current to said coilproportional to the displacement of the conducting planar element, saidmeans for applying excitation to the inductive pickoif elementconsisting of an oscillator which normally oscillates continuously, theoscillator including a tank circuit and a feedback circuit, one of thewindings of the inductive pickoff element being connected into the tankcircuit of the oscillator to form a part of the tank circuit and theother of the windings being connected into the feedback circuit of theoscillator, the output of the oscillator being modulated by variationsin spacing between the conducting element and the inductive pickofielement, and additional feedback means coupling the detector means tothe oscillator to change the gain of the oscillator to thereby controlthe loop gain and the frequency response of the transducer.

2. In a transducer for measuring linear acceleration, a housing havingends, a mass, means supporting said mass within said housing for linearmovement toward and away from said ends, an inductive pickoff elementhaving two windings mounted on one of said ends of the housing, aconducting planar element mounted on the end of said mass adjacent saidpickofi element and oh one side of the pickofli element, a magnetmounted on the other end of said housing, a coil mounted on the otherend of said mass adjacent and in the field of said magnet, oscillatormeans for applying excitation to the pickofl element to establish amagnetic field, the conducting planar element being normally positionedrelatively close to the pickoff element so that losses are reflectedinto the oscillator means by the conducting planar element to change theloop gain of the oscillator means, detector means connected to saidoscillator means to give an indication of the position of the conductingplanar element relative to the pickoff elemente, [and] feedback meansconnected to said detector means for applying current to said coilproportional to the displacement of the conducting planar element, saidoscillator means for applying excitation to the pickoff elementconsisting of an oscillator including a tank circuit and a feedbackcircuit, one of the windings of the pickoif element being connected tothe tank circuit of the oscillator to form a part of the tank circuitand the other of the windings being connected into the feedback circuitof the oscillator, the output of the oscillator being amplitudemodulated by variations in spacing between the conducting planar elementand the pickoif element which causes a change in the Q of the tankcircuit of the oscillator, and additional feedback means in the form ofa resistive-capacitive network coupling the detector means to theoscillator to change the gain of the oscillator to thereby control theloop gain and frequency response of the transducer.

3. In a transducer, a rigid framework, a torsion member mounted in saidrigid framework, a mass supported by said torsion member, a conductingplanar element carried by the mass, a pickolf transformer mounting onthe framework and disposed adjacent the conducting planar element, thepickoif transformer consisting of a pair of windings disposed on oneside of the conducting planar element, oscillator means for applyingexcitation to the pickoif transformer to establish a magnetic field, theconducting planar element and the pickoif transformer being positionedso that the conducting element disturbs the coupling between thewindings and introduces losses into the oscillator means to change theloop gain of the oscillator, and detector means connected to saidoscillator means to give an indication of the position of the conductingelement relative to the transformer, said oscillator means for applyingexcitation to the pickotf transformer consisting of an oscillatorincluding a tank circuit and a feedback circuit, one of the windings ofthe pickolf transformer being connected in the tank circuit of theoscillator to form a part of the tank circuit and the other of thewindings being connected into the feedback circuit of the oscillator,the Q of the tank circuit being modulated by variations in spacingbetween the conducting planar element and the pickofi transformer andfeedback means coupling the detection means to the oscillator to changethe gain of the oscillator to thereby control the loop gain andfrequency response of the transducer.

4. A transducer as in claim 3 together with an additional conductingplanar element fixed to the mass on the opposite side of the mass andextending in the same plane as the first named conducting planarelement, an additional pickoff transformer mounted on said framework anddisposed adjacent said additional conducting planar element on the sideof said additional conducting planar element opposite the side on whichsaid first named pickoff transformer is mounted with respect to thefirst named conducting element, the first named and additional pickoiftransformers being arranged so that upon rotational movement of the massin one direction, both of said conducting planar elements will be movedcloser to their respective pickolf transformers and upon rotation ofsaid mass in an opposite direction both of said conducting planarelements will be moved away from their respective pickoff transformers.

5. In a transducer, a conducting planar element, a pickoff transformerconsisting of a pair of windings disposed on one side of the conductingplanar element, an oscillator [circuit] including a tank circuit and afeedback circuit, one of the windings of the transformer being connectedinto the tank circuit of the oscillator to form a part of the tankcircuit and the other being connected into the feedback circuit of theoscillator, the coupling between the windings and the Q of the tankcircuit being changed and the output of the oscillator being modulatedby variations in spacing between the conducting element and the pickofftransformer, [and] detector means connected to the oscillator to give acontinuous indication of the position of the conducting element relativeto the pickoif transformer, and feedback means coupling the detectormeans to the oscillator to change the gain of the oscillator to therebycontrol the loop gain and frequency response of the transducer.

6. A transducer as in claim 5 wherein the conducting element is movablewith respect to said pickoif transformer together with restoring meansfor placing a restoring force on the conducting element proportional tothe displacement of the conducting element.

7. A transducer as in claim 5 wherein said oscillator circuit includes atransistor having base, collector and emitter elements, and wherein saidwinding connected in the tank circuit is connected between the base andcollector elements of said transistor and said Winding connected to thefeedback circuit of the oscillator is connected between the base and theemitter elements of the transistor being arranged in a grounded baseconfiguration and operating with an alpha gain.

8. A transducer as in claim 5 wherein the winding connected to thefeedback circuit of the oscillator has an impedance of intermediatevalue so that the output of the oscillator remains unchanged throughrelatively wide temperature ranges.

9. In a transducer, a conducting planar element, a p-ickoif transformerconsisting of a pair of windings, an oscillator circuit including a tankcircuit and a feedback circuit, one of the windings of the transformerbeing connected into the tank circuit of the oscillator to form a partof the tank circuit and the other being connected into the feedbackcircuit of the oscillator, the output of the oscillator being modulatedby variations in spacing between the conducting element and the pickoiftransformer, detector means connected to the oscillator to give anindication of the position of the conducting element relative to thepickoff transformer, the conducting element being movable with respectto the pickup transformer, and restoring means for placing a restoringforce on the conducting element proportional to the displacement of theconducting element, said restoring means consisting of a compliantelastic member connected to the conducting element, and means forfeeding back a signal from said detector means to the feedback circuitof the oscillator to thereby affect the operating characteristics of theoscillator, the signal having a value so that the average D.-C. outputfrom the detector means is zero.

10. In a transducer, a conducting planar element, a pickoff transformerconsisting of a pair of windings positioned relatively close to theconducting element, an oscillator circuit having a tank circuit and afeedback circuit, one of the windings of the transformer being connectedinto the tank circuit of the oscillator to form a part of the tankcircuit and the other being connected to the feedback circuit of theoscillator, the output of the oscillator being modulated by variationsin spacing between the pickoff transformer and the conducting element,detector means connected to the oscillator for giving an indication ofthe position of the conducting element relative to the pickoiftransformer, amplifier means connected to said detector means, andfeedback means connected between the output of the amplifier and thefeedback circuit of the oscillator to control the operating level of theoscillator and to thereby maintain the average D.-C. output from theamplifier at zero.

11. In a transducer, a conducting planar element, an inductive pickoffelement positioned relatively close to the conducting planar element, anoscillator circuit having a tank circuit and a feedback circuit, theinductive pickoif element being connected to the tank circuit of theoscillator to form a part of the tank circuit, the Q of the tank 19:ircuit being changed and the output of the oscillator )eing modulatedby variations in spacing between the :onducting planar element and theinductive pickoff elenent, detector means connected to the oscillatorand givn-g an indication of the position of the conducting planar:lement relative to the inductive pickoff element, and 'eedback meanscoupled between the output of the deector means and the feedback circuitof the oscillator to :ontrol the operating level of the oscillator andto thereby naintain the average D.-C. output from the detector neans atZero.

12. In a transducer for measuring acceleration, a conlucting element,means for mounting said conducting :lement for movement, a magnet, acoil disposed in the nagnetic field of said magnet and secured to saidconlucting element for movement with said conducting elenent, aninductive pickoif element having a pair of windngs disposed on one sideof the conducting element, an )SCiIIatOI' having a tank circuit andfeedback means, one if the windings of the inductive pickoif elementbeing :onnected into the feedback means of the oscillator, the)scillator serving to supply self-generating excitation to he inductivepickoif element to establish a magnetic field, he conducting elementbeing positioned relatively close the inductive pickotf element 30 thatit varies the load mpedance on the oscillator to thereby modify the loopgain of the oscillator, the loading of the oscillator by the novement ofthe conducting element serving to inhibit )scillation of the oscillator,detecting means connected 0 said oscillator to give an indication of theposition of he conducting element relative to the inductive pickoff:lement, [and] feedback means coup-led to said detector neans forapplying a current to said coil proportional to he displacement of theconducting element, and negative eedback means in the form of aresistive-capacitive netvork coupling the detector means to theoscillator to conrol the loop gain and the frequency response of thetransiucer.

13. In a transducer for measuring acceleration, a conlucting element,means for mounting said conducting elenent for movement, a magnet, acoil disposed in a magietic field of said magnet and secured to saidconducting :lement for movement with said conducting element, annductive pickoif element, an oscillator having a tank cir- :uit andfeedback means, the inductive pickolf element )eing connected to andforming a part of the tank circuit )f the oscillator, the oscillatorserving to supply self- ;enerating excitation to the inductive pickoifelement to :stablish a magnetic field, the conducting element being:ositioned relatively close to the inductive pickoff element to that itvaries the load impedance on the oscillator to hereby modify the loopgain of the oscillator, the loading )f the oscillator by the movement ofthe conducting elenent serving to inhibit oscillation of the oscillator,deector means connected to said oscillator to give an indi- :ation ofthe position of the conducting element relative o the inductive pickoffelement, and feedback means :oupled to said detector means for applyinga current to :aid coil proportional to the displacement of the conductngelement, said oscillator including a transistor having a )ase, emitterand collector elements, and an emitter base lriving circuit, the tankcircuit forming a part of the :mitter base driving circuit of thetransistor, the impedll'lCe presented to the emitter base drivingcircuit of the ransistor by the inductive pickoif element and the tank:ircuit being chosen so that the voltage gain and the curent gain of thetransistor are utilized to achieve a con- .tant gain for the transistorindependent of temperature.

14. In a transducer, a conducting element, a pickolf ransformerconsisting of a pair of windings disposed n relatively close proximityto and on one side of the :onducting element, an oscillator having atank circuit 1nd a feedback circuit, one of the windings of thetransformer being connected into the tank circuit of the oscilator toform a part. of. the ank circuit. and the other of the windings beingconnected to the feedback circuit of the oscillator, the conductingelement and the pickoif transformer being arranged so that upon a changein spacing between the conducting element and the pickoff transformer,two aiding effects occur to modulate the output of the oscillator inaccordance with the variations in spacing between the pickotftransformer and the conducting element, one of the effects being thechange in coupling between the two windings, the other effect being tochange the Q of the tank circuit, [and] detector means connected to theoscillator for giving an indication of the spacing between theconducting element and the pickofi' transformer, and negative feedbackmeans in the form of a resistive-capactive network connected between thedetector means and the oscillator.

15. A transducer as in claim 14 wherein said pair of windings arecoaxial and of the pancake type and lie in a plane substantiallyparallel to the plane of the conducting element.

16. A transducer as in claim 14 wherein the pair of windings and theconducting element are arranged so that as the spacing between theconducting element and the pickotf transformer is decreased, thecoupling between the two windings is decreased and the Q of the tankcircuit is lowered.

17. In a transducer, a conducting planar element, a pickoif transformerconsisting of a pair of coaxial windings, the windings lying in a planerelatively close to and substantially parallel to the plane of theconducting element, an oscillator [circuit] having a tank circuit and afeedback circuit, one of the windings of the transformer being connectedinto the tank circuit of the oscillator to form a part of the tankcircuit and the other being connected into the feedback circuit of theoscillator, the output of the oscillator being modulated by variationsin spacing between the pickofi transformer and the conducting element,detector means connected to the oscillator for giving an indication ofthe position of the conducting element relative to the pickolftransformer, and amplifier means connected to said detector means, saidamplifier means including first, second and third transistors havingbase, collector and emitter elements, means for connecting the output ofthe detector means to the base of the first transistor, means connectingthe emitter of the first transistor to the base of the secondtransistor, means for connecting the collector of the first transistorto the base of the third transistor, a negative D.-C. supply, means forconnecting the negative D.-C. supply to the emitter of the secondtransistor, a positive D.-C. supply, means for connecting the positiveD.-C. supply to the collector of the third transistor, impedance meansconnecting the collector of the second transistor to the emitter of thethird transistor, impedance means connecting the positive D.-C. supplyto the base of the third transistor, [and] diode means connected betweenthe collector of the second transistor and the base of the thirdtransistor, a load resistor connected between the emitter of the thirdtransistor and ground and negative feedback means in the form of aresistive-capacitive network coupled to the load resistor and connectedto the oscillator.

18.A transducer as in claim 17 together with a constant [a load resistorconnected between the emitter of the third transistor and ground, andfeedback means coupled to said load resistor] voltage power supply meansconnected to said oscillator.

19. A transducer as in claim [18] 17 wherein said feedback means isconnected into the feedback circuit of the oscillator to thereby controlthe operating level of the oscillator and to thereby maintain theaverage D.-C. output from the amplifier at zero.

20. A transducer as in claim 18 together with restoring means connectedto said conducting element and [wherein] said additional feedback means[is] connected to said restoring means and coupled to the load resistor,

References Cited 5 The following references, cited by the Examiner, areof record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 10 Bradenburger.

Gilbert.

Roper.

Greenough.

Ter Veen et al. 1 15 3/ 1958 Cloud.

8/1959 Driver.

6/1960 Turner. 10/ 1953 Sheppard 73-517 12/ 1954 Stanton 73-517 8/1958Marggrad 73-517 11/1959 Schaevitz 73517 3/1962 Bonnell 73517 FOREIGNPATENTS 9/1954 Great Britain.

RICHARD C. QU-EISSER, Primary Examiner H. GOLDSTEIN, Assistant Examiner

